1. Field of the Invention
The invention relates to a method as described herein and also relates to a bending die as described herein.
2. Description of the Related Art
For a long time, the bending of workpieces has been a frequently applied and reliable method for processing workpieces by reshaping. The scope of application of bending processes is frequently limited by material properties, especially by the mechanic-technological properties. The problem concerning brittle materials like magnesium, titanium, spring steels, high-strength aluminum-alloys, high-strength steels or other materials known to be brittle is that in case of a deforming by bending, these materials do not provide sufficient plastic formability and thus other undesired deformations appear. A parameter that can indicate the respective behavior of materials is the so-called ultimate strain that means the value of the plastic deformation that a workpiece to be deformed can bear until it breaks. An alternative parameter for this behavior is also the so-called yield strength to tensile strength ratio that considers the tension required in a workpiece at the beginning of a noticeable plastic deformation in relation to the tension within the workpiece in case of breaking load.
In order to make such materials having a low ultimate strain or a high elasticity ration accessible to the application of a deformation methods, especially to bending, methods putting the workpiece into a condition providing more favorable mechanic characteristics and enabling it to be deformed by means of a bending method have been applied for a while. A known method is heating the workpiece to be bent at least in the region of the deformation zone, with the result that in this heated area the tension necessary for initiating of plastic deformation can be reduced.
As an example of such a method, the EP 0 993 345 A1 discloses a method for bending a workpiece by application of mechanic force under selective heating of the workpiece along a bending line by means of laser radiation, where one laser beam or several laser beams are formed to be an elongate radiation field and where a heating zone along the bending line of the workpiece is created by the radiation field. In this case, the device for forming the linear radiation field comprises cylindrical lenses and/or cylindrical mirrors, which are used to guide a radiation field through the opening in the bending die onto the workpiece. In the exemplary embodiment according to FIG. 4 of the EP-A1, a laser beam is split into two radiation fields by means of a beam forming optic, which consists of a prism mirror, two cylindrical lenses and two cylindrical passive deflectors. The two radiation fields are guided through the bending die onto the workpiece and produce respective linear heating zones. The laser beam deformed this way is thus guided onto the workpiece through a slot-like opening in the bottom side of the die.
This solution for the guiding of high-energy radiation in a bending die known from the EP 0 993 345 A1 is not ideally suited for the practical application with common bending machines, because the bending die provides a limited mechanical strength due to its two-piece embodiment and the press beam receiving the bending die would have to provide recesses for the beam distribution arrangement.